1. Field of the Invention
The invention relates to active halogen containing bleach particles and a method for bleaching substrates through slow uniform release of halogenating agent.
2. The Prior Art
Particles containing oxidants for bleaching substrates have been widely disclosed in the literature. Much research has focused upon coating or encapsulating chlorinating agents, e.g. dichloroisocyanurate granules, to obtain delayed, slow release of active oxidant.
When used for cleaning clothes in automatic washing machines, several problems are noted with encapsulated oxidants. Low bleaching strength is encountered because of incomplete dissolution of the encapsulates during the standard wash cycle. Another problem is severe fabric color damage from the localization of released bleach. Generally, bleaching products are placed into the automatic washing machine simultaneously with the dry load. Bleach and fabric remain in close contact as the machine fills with water. Local high concentrations of bleaching actives thereby come into contact with fabric surfaces. Under these conditions, very small spots resembling pinholes appear on the fabric.
U.S. Pat. No. 4,136,052 (Mazzola) reports to have solved the pinhole problem caused by localized high concentrations of bleach. The patent provides a special coating which encapsulates the bleaching compound. An active chlorinating agent is surrounded by a first non-reactive coating combination of fatty acid and wax. A second coating is applied containing fatty acid with a material exhibiting inverse aqueous solubility with respect to temperature. The outer, second coating is more resistant to dissolution in hot than in cold water. By this means, sufficiently delayed bleach release is provided in hot water to prevent pinholing.
U.S. Pat. No. 3,908,045 (Alterman et al.) discloses dichloroisocyanurate salts encapsulated with a first coating of a saturated fatty acid surrounded by a second coating of soap. The latter coating is formed by treatment of portions of the inner fatty acid shell with a solution of an alkali metal hydroxide.
Organic coating materials protect the bleaching agent in the foregoing patents. A slightly different approach is reported in U.S. Pat. No. 3,112,274 (Morgenthaler et al.). Inorganic salts such as sodium tripolyphosphate are applied in a fluidized bed to coat polychloroisocyanurate salts. The resultant encapsulated salts are said to be protected from decomposition by the attack of moisture, air or other reactive materials.
The prior art coatings surrounding the chlorine bleach provide adequate protection against pinhole type fabric damage only at low and medium wash temperatures. Unfortunately, at hot wash temperatures above the melting point of the coating materials, pinholing is still a problem. It has been suggested that hot water pinholing is greatly increased by non-uniform coating. Fabric damage is caused here by the inadequately encapsulated particle fraction. To solve the problem, average coating weights have been increased by as much as 50% over the known art. Even these increased thicknesses do not ensure complete absence of pinholing at hot wash temperatures. Very thick coatings, which do control pinholing, are deficient because they nearly eliminate chlorine release at low wash temperatures.
Consequently, it is an object of the present invention to provide bleach particles which eliminate pinholing yet have satisfactory active halogen release at all wash temperatures.
A further object of this invention is to provide bleach particles of spherical shape that are readily coatable to a thickness substantially uniform for all particles.
Another object of this invention is to provide a method for bleaching various substrates including fabrics.
Another object of this invention is to establish a reliable, efficient method for production of these bleach particles.